This invention relates to a method for controlling a transfer case and more particularly, to a method for controlling a transfer case within a four wheel drive vehicle which provides a relatively quick and smooth torque adjustment response by utilizing both a proportional and an integral control component.
Four-wheel drive vehicles typically include a transfer case which selectively transfers torque and power from the vehicle""s input shaft to a front and a rear driveshaft, thereby selectively rotating the front and rear driveshafts. The transfer case includes a conventional electromagnetic clutch assembly which typically and selectively transfers torque from a primary driveshaft (e.g., the rear driveshaft) to a secondary driveshaft (e.g., the front driveshaft), thereby reducing the torque provided to the primary driveshaft and increasing the torque provided to the secondary driveshaft. The clutch assembly is typically and communicatively coupled to a controller and/or control system which determines the amount of torque that is to be provided to the rear and front driveshafts (e.g., primary and secondary driveshafts), based upon vehicle sensory data, and which generates a control or command signal to the clutch assembly, thereby controlling the amount of torque which is provided to front and rear driveshafts.
In order to provide torque and traction to the front and rear wheels of the vehicle and to substantially avoid relative slip between the front and rear wheels, the transfer case control system typically monitors and adjusts the speed of both the front and rear driveshafts. For example and without limitation, when the speed of the front driveshaft or front wheels differs from the speed of the rear driveshaft or rear wheels by and an amount or xe2x80x9cerrorxe2x80x9d, these prior systems alter or change the value of the control or command signal to provide more or less torque to the front and/or to the rear driveshaft in order to substantially prevent and/or reduce the likelihood of relative slip between the front and rear wheels. Particularly, these prior systems typically alter the magnitude of the control signal based upon the difference between the speed of the front and rear driveshafts (e.g., the xe2x80x9cdelta shaft speedxe2x80x9d), and typically xe2x80x9cstep upxe2x80x9d or xe2x80x9cstep downxe2x80x9d the magnitude of the control signal based on whether the delta shaft speed exceeds a slip table value.
While these types of prior systems relatively quickly respond to and correct relative slip conditions between the front and rear wheels in many situations, they suffer from some drawbacks. One drawback associated with these prior systems is that the xe2x80x9cdelta shaft speedxe2x80x9d value may vary significantly over relatively short periods of time, thereby causing the value of the command signal to vary significantly over the same period of time. This variance of the command signal often results in a xe2x80x9croughxe2x80x9d or xe2x80x9cchoppyxe2x80x9d ride and in undesirable noise and vibration harshness (xe2x80x9cNVHxe2x80x9d) problems, which are experienced by the operator and/or passengers of the vehicle.
Furthermore, these prior strategies that xe2x80x9cstepxe2x80x9d the clutch response by a fixed amount do not account for the degree and duration of slip. Particularly, these strategies can lead to a slow response in cases of sudden, dramatic slip (such as wheels on ice), as the controller must gradually build up a clutch response. The constant rate can also lead to an objectionable rhythmic pulsation when the vehicle is on a consistently slippery surface (e.g., sand), as the system constantly slips, ramps up the clutch, resolves slip, ramps down the clutch, then slips again and repeats the process at a consistent rate.
There is therefore a need for a method and an apparatus for controlling a transfer case within a four-wheel drive vehicle which provides a relatively quick and smooth torque adjustment response by utilizing both a proportional and an integral control component.
It is a first object of the invention to provide a method and an apparatus for controlling a transfer case within a four wheel drive vehicle which overcomes at least some of the previously delineated drawbacks of the prior systems, devices, and/or methods.
It is a second object of the invention to provide a method and an apparatus for controlling a transfer case within a four-wheel drive vehicle which monitors relative slip between the front and rear wheels of the vehicle and which provides a relatively smooth and quick torque adjustment response to and correction of relative slip situations.
It is a third object of the invention to provide a method and an apparatus for controlling a transfer case within a four-wheel drive vehicle which includes a proportional control component and an integral control component and which is effective to substantially prevent slip between the front and rear wheels of the vehicle in a manner which does not cause undesirable NVH.
According to one aspect of the present invention a system for controlling a transfer case is provided. The transfer case is operatively disposed within a four-wheel drive vehicle of the type having a front driveshaft which rotates at a first speed during a first interval of time and at a second speed during a second interval of time, and a rear driveshaft which rotates at a third speed during the first interval of time and at a fourth speed during the second interval of time. The transfer case is adapted to selectively provide torque to the front and the rear driveshaft. The system includes a first sensor which measures the first speed and generates a first signal which represents the first speed, and which measures the second speed and generates a second signal which represents the second speed; a second sensor which measures the third speed and generates a third signal which represents the third speed, and which measures the fourth speed and generates a fourth signal which represents the fourth speed; and a controller which is communicatively coupled to the transfer case and to the first sensor and the second sensor. The controller receives the first, second, third and fourth signals, utilizes the first signal and the third signal to calculate a first error value, and utilizes the second signal and the fourth signal to calculate a second error value. The controller is further adapted to selectively generate an output signal which is derived based upon the first error value and the second error value, the output signal being effective to selectively control said torque which is transferred to the front and the rear driveshaft.
According to a second aspect of the present invention, a method for controlling a transfer case which is operatively disposed within a four wheel drive vehicle of the type having a front driveshaft rotating at a first speed and a rear driveshaft rotating at a second speed, is provided. The transfer case is adapted to selectively provide torque to the front driveshaft and the rear driveshaft. The method includes the steps of: calculating an error value based upon a difference between the second speed and the first speed; calculating a proportional value which is proportional to the error value; calculating an integral value which is based upon the error value and a previously calculated integral value; generating an output signal, the output signal having a value equal to the proportional value plus the integral value; and selectively transmitting the output signal to the transfer case, effective to control the amount of torque provided to the front and the rear driveshaft.
Further objects, features, and advantages of the invention will become apparent from the following detailed description of the preferred embodiment of the invention and by reference to the following drawings.